Electrophotographic imaging processes and techniques have been extensively described in patents and other literature. The initial image forming step in the electrophotographic process cycle is the creation of an electrostatic latent image on the surface of a photoconductive element. This can be accomplished by charging the photoconductive element in the dark, such as through use of a corona or biased roller charging element. An electrostatic latent image is then formed by image-wise exposing the photoconductive element either optically or by electronic means, such as a laser or an array of light-emitting diodes. The image exposure creates free electron-hole pairs which migrate through the photoconductive element under the influence of the electric field. In such a manner, the surface charge is dissipated in the exposed regions, thus creating an electrostatic charge pattern. A visible image is then formed by depositing electrophotographic toner, comprised of electrically charged marking particles, onto the electrostatic latent image during a development step.
Two methods of development are used in electrophotography: discharged area development (DAD) and charged area development (CAD). The former uses toner of the same polarity as the surface charge on the photoconductive element. The latter utilizes toner of polarity opposite to the polarity of the charge on the element. CAD is widely used in optical copiers, while DAD is more frequently used for digital applications.
The image formed in the development step is transferred to a suitable receiver, such as transparency stock or paper. It can be transferred to a receiver directly, with the assistance of, for example, an electric field or through the application of heat, pressure, or heat and pressure. Alternatively, the image can be first transferred to an intermediate member and, subsequently, transferred to the receiver. Color images can be made by transferring images comprised of the primary colors, (i.e., cyan, magenta, yellow, and black) in register, to either the receiver or the intermediate member.
After transfer, the image is permanently fused to the receiver via a suitable fusing process. In preparation for the next electrophotographic cycle, the photoconductive element is typically cleaned of residual toner because the transfer step is not 100% efficient. Cleaning efficiency is increased by electrostatically conditioning the photoconductive element and residual toner with a charging element known as a pre-clean charger. Cleaning by any of a number of methods known to one skilled in the art is then performed.
Photoconductive elements, also called photoreceptors, are composed of an electrically conductive support and at least one active layer which is insulating in the dark but which becomes conductive upon exposure to light. The support may be in one of many forms, for example, a drum, a web or belt, or a plate. The photoreceptor can comprise one or multiple active layers. The active layer(s) typically contains one or more materials capable of the photogeneration of charge carriers (electrons or holes) and one or more materials capable of transport of the generated charge carriers.
Numerous materials have been described as being useful components of the photoconductive element. These include inorganic substances, such as selenium and zinc oxide, and organic compounds, both monomeric and polymeric, such as arylamines, arylmethanes, carbazoles, pyrroles, phthalocyanines, dye-polymer aggregates, and the like. Organic compounds are particularly useful for several reasons. They can be prepared as flexible layers; as such, the copier or printer architecture is not limited to a particular configuration. Organic compounds have spectral sensitivities that can extend throughout the visible and into the near infrared regions of the spectrum. Organic compounds are amenable to low cost large area manufacturing processes. Elements prepared from organic materials are known as organic photoconductors (OPCs).
OPCs can be prepared with single or multiple active layers. In most OPCs, charge transport occurs through movement of a single type of charge carrier, electrons or holes, but not both. When only one carrier is mobile, trapped carriers of opposite sign can be created, resulting in a change in sensitometry of the active layer with succive cycles, a phenomenon known as latent image hysteresis. One solution to the problem of latent image hysteresis is to separate the charge generation and transport functions into separate layers, referred to as the charge generation (CGL) and charge transport (CTL) layers, to form a dual or multi-layer photoconductive element. These elements offer additional advantages of improved process lifetimes that make them useful in high volume copying and printing applications. A disadvantage of the multiple layer architecture is that only one polarity of surface potential may be employed, limiting the electrophotographic processes in which the element can be used.
Few bipolar charge transport materials (CTMs) capable of transporting both carrier types are known. The known materials have several disadvantages. They are composed of complexes of at least two separate molecules. The necessity to form the complexes increases both the complexity of the manufacture of photoconductive elements and the likelihood that imperfectly formed complexes will give rise to defect points or nonuniformities in a photoconductive layer of the elements. Also, the complexes are not stable over long periods of time. Thus, there is a need for a bipolar CTM that is a single material, a molecular bipolar CTM, that is useful in photoconductive elements used in electrophotography.
Only one molecular bipolar charge transport material capable of transporting both electrons and holes is known. Murray et al. (B. J. Murray, J. E. Kaeding, W. T. Gruenbaum, and P. M. Borsenberger, Jpn. J. Appl. Phys. 1996, 35, 5384-5388) reported that N-(p-(di-p-tolylamino)phenyl)-N'-(1,2-dimethylpropyl)-1,4,5,8-naphthalenet etracarboxylic diimide (TAND), in combination with an amorphous selenium CGL, could transport both electrons and holes. Amorphous selenium is undesirable for practical application in a photoconductive element because of the hazards associated with its deposition and disposal. Further, its spectral sensitivity extends to only 500 nm, too low for applications using laser or LED exposure. Kaeding et al. (J. E. Kaeding, B. J. Murray, W. T. Gruenbaum, and P. M. Borsenberger, J. Imag. Sci. Technol. 1996, 40, 245-248) further reported bipolar transport by TAND in combination with an unspecified perylene diimide. While the electron and hole mobilities of TAND are analyzed in this paper, no mention is made of the electrophotographic properties of a photoconductive element containing TAND in combination with a CGL that would be useful in an electrophotographic process. The information in these papers is insufficient to determine the usefulness of TAND as a bipolar CTM in a photoconductive element for use in an electrophotographic process.
The electrophotographic properties that a useful bipolar CTM must impart to a photoconductive element are high sensitivity and low residual voltage. The sensitivity is characterized by the exposure energy (E.sub.50%), the energy required to discharge the photoconductive element from an initial potential to a final potential that is half the initial potential, for example from an initial potential of 350 V to a final potential of 175 V. Higher exposure energies indicate a less sensitive photoconductive element, one in which higher energy exposures would be required to generate the latent image. The residual voltage (V.sub.r) is a measure of the charge remaining on the element after exposing the element and allowing the surface potential to discharge. High residual voltages can give rise to lower potential differences between charged and discharged areas of the element on subsequent imaging cycles. Blurred, fogged, or incomplete images can result. For high process efficiency, low exposure energies and low residual voltages are desired.
A useful bipolar CTM must be capable of producing a photoconductive element that displays the desirable electrophotographic properties under both polarities of initial surface charge. A useful bipolar CTM can be defined as one in which, when incorporated into a photoconductive element, results in a specific ratio of the exposure energies of the element measured under positive and negative polarity initial charging. Specifically, if the exposure energy of the element after positive polarity charging is denoted E.sup.+.sub.50% and if the exposure energy of the element after negative polarity charging is denoted E.sup.-.sub.50%, then a bipolar CTM is one which produces a photoconductive element having .alpha.=E.sup.+.sub.50% /E.sup.-.sub.50%, where .alpha. is between 0.25 and 4.0. Both E.sup.+.sub.50% and E.sup.-.sub.50% are measured in erg/cm.sup.2 and measure the energy necessary to discharge the photoconductive element from an initial voltage (e.g., 350 V) to half the initial voltage (e.g., 175 V).